Epidemiology

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Epidemiology
Molecular epidemiology, Microbial Ecology,
Vector Ecology and Population Biology
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Tracking microorganisms through space

• Spatial distribution of organisms may depend on
• Geographic range
• Habitat specificity
• Host specificity
• Ecosystem specificity
• Site or tissue specificity

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Microbes are everywhere

• Microbes are found all over the world in
  virtually every habitat and in/on most organisms
  as symbionts
• Free living microbes may sometimes become
  opportunistic parasites
• Humans non-specific defenses help to protect
  from these opportunists
• Some microbes are specialized pathogens and have
  specific abilities and requirements to cause
  infections

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Transport and Transmission of microbes

• Abiotic routes, vehicles and fomites (indirect
  contact)
• Water
• Wind/thermal currents
• Food
• Other
• Biotic routes
• Host to host (Direct contact)
• Human to human
• Animal reservoir to human
• Vector
• Biological (blood or feces of vector)
• Mechanical (vector transports on body)
• Combination of biotic and abiotic

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Microbes in Water
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Microbes in Water
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Contamination of Water affects drinking and
recreational water

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Food Borne Illnesses

• Fecal contamination of food
• From animals (E. coli, Salmonella etc)
• From humans (hepatitis A etc)
• Parasites in tissue of meat (helminthes etc)
• Food spoilage or intoxication (Botulism,
  aflatoxins etc..)

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Direct Contact Transmission

• Types of contact Bites, scratches, sneezes,
  coughs, sexual contact, physical contact,
  exposure to blood or body fluids
• Direct contact with
• With humans-examples include VDs (STDs), MRSA
• With animals examples include rabies, ringworm
• With vehicles (blood, feces, body fluids etc)
  brucellosis, Q fever, Tularemia

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Vector Transmission

• Vectors are usually arthropods
• Biological transmission by haematophagous vectors
  (blood feeding vectors)
• Through saliva of vector
• dirty needle transmission by vectors proboscis
  (rarely occurs)
• Through blood or feces of vector
• Accidental ingestion of vector
• Mechanical vectors (non blood feeding)
• On body of vector
• In feces of vector
• Accidental ingestions

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Insects

• Siphonaptera-fleas
• Diptera-flies
• Nematocera
• Ceratopogonidae-no-see-ums, sand gnats
• Culicidae-mosquitoes
• Psychodidae-sand flies
• Simuliidae-blackflies
• Brachycera
• Tabanidae-horse and deer flies
• Cyclorrhapha
• Muscidae-houseflies
• Glossinidae-tetse flies
• Hemiptera-bugs
• Reduviidae-conenose (kissing) bugs
• Cimicidae-bed bugs
• Dictyoptera
• Blattidae-roaches
• Anoplura-sucking lice

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Some Insect Vectors
Cone nose bug
Mosquito
Flea
Tsetse fly
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Mites (including ticks)

• Arachnida
• Acari
• Ixodidae-hard ticks
• Argasidae-soft ticks
• Laelapidae-hematophagous mites
• Dermanyssidae-hematophagous mites
• Demodicidae-follicle mites
• Trombiculidae-chiggers
• Sarcoptidae-scabies mites

Ticks
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Ticks

• Ticks are important vectors
• Some species also cause tick paralysis

American dog tick
Lone star tick
Black legged tick (deer tick)
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Examples of vector-borne diseases
Mosquitoes- malaria, hemorrhagic fevers, viral
encephalitis, filariasis Black flies- river
blindness Sand flies- leishmaniasis, Oroya
fever Fleas-plague, maybe bartonellosis, some
tapeworms Lice- trench fever, epidemic typhus,
relapsing fever Deer flies- loaiasis Conenose
(kissing) bugs- Chagas disease Tsetse flies-
African sleeping sickness Ticks- Lyme disease,
ehrlichiosis, RMSF, hemorrhagic fevers Chiggers-
scrub typhus Copepods and other crustaceans-
cholera, dracunculiasis
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Reservoirs

• Reservoirs maintain the disease agent in nature
• Animals
• Vertebrates- deer, rodents, monkeys etc..
• Invertebrates- clams, crustaceans, insects
  (remember, sometimes a vector can also be a
  reservoir)
• Protists- some amoebas
• Water or soil- some free-living, infectious
  organisms can reproduce outside of the host in
  water or soil (otherwise water and soil are just
  vehicles)

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Natural factors that affect distribution and
abundance of disease-causing organisms

• Environmental changes
• Individual host, vector or reservoir
• Population of hosts, vectors or reservoirs
• Microbial antagonism, competitive exclusion
• Genetic changes in disease agent
• Natural selection
• Stochastic effects (e.g. drift, bottlenecks etc)
• Host health and immunity (e.g. stress)

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Environment and Disease
Host availability and abundance (including
vectors and reservoirs)
E
Ecosystem balance
Abiotic factors (Weather)
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Anthropogenic and societal factors that influence
disease

• Environmental degradation

Pollution
Poverty
Education
Hygiene
Overcrowding
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Studying Temporal Variation

• The distribution and abundance of microorganisms
  may vary with time
• Outbreaks of disease may occur periodically in
  relation to changes in the environment, changes
  in hosts, genetic change in the microbe itself,
  or a combination of factors
• For example, each year, outbreaks of influenza
  result in about 30,000 deaths. However, the
  severity may differ over time as different
  strains emerge

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Hospitals and the spread of disease

• Nosocomial infections occur in hospitals
• Hospitals may contain more immunocompromised
  people, who are more susceptible
• Invasive procedures are conducted, creating
  portals of entry for pathogens
• Many opportunistic pathogens
• Frequent use of antibiotics and the potential
  spread of antibiotic resistance genes
• Misdiagnosis of disease especially emerging or
  obscure pathogens

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Pathogens frequently causing nosocomial infections

• Staphylococcus aureus and MRSA
• Streptococcus spp.
• Pseudomonas aeruginosa
• Enterococcus spp.
• Escherichia coli
• Klebsiella spp.
• Enterobacter spp.
• Clostridium difficile
• Candida albicans
• Many others

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Epidemiology and Infection Terminology

• Acute- short-term infection with dramatic onset
  and rapid recovery (or death)
• Chronic- long-term infection
• Definitive host- host in which a parasite reaches
  sexual maturity
• Endemic-naturally occurring in a particular area
• Enzootic-presence of pathogen in particular area
  maintained by local reservoirs and vectors
• Epidemic-presence of disease agent above normal
  infection prevalence
• Epizootic-out break of enzootic pathogen
• Etiology- cause of disease
• Fomite-inanimate objects that can transmit
  pathogens between hosts
• Incidence-the number of new host who become
  infected
• Infectious dose- number of agents required to
  cause disease
• Intensity- the number of parasites in a single
  host
• Intermediate host- host in which parasite
  develops to some extent but not to sexual
  maturity
• Microbial antagonism- competition between
  microbes which can lead to suppression of a
  particular type of disease agent

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• Opportunistic Pathogen/parasite (phoront or
  commensal becomes parasitic)
• Obligate Parasite (cannot live without host)
• Pathogen- organism that causes disease
• Pathogenicity- organisms ability to cause disease
• Prevalence-the rate or frequency of an organism
  as a proportion or percent
• Reservoir-host that maintains disease agent in
  nature
• Resident- symbiont that remains in a host for a
  significant period of time
• Sylvatic-exist normally in the wild, not in the
  human population
• Transient- symbiont that is in a host temporarily
• Transovarial transmission-vertical
  transmission-vector to offspring
• Transstadial transmission-transmission of agent
  across life stages of vector
• Vector- Organism that carries an agent from one
  host to another
• Vehicle- inanimate source of pathogens
• Virulence-degree of pathogenicity of an organism
• Zoonosis-disease of animals that can be
  transmitted to humans
• Suffixes -emia presence of, -osis, -iasis, -itis
  - referring to disease state or condition of
  infection